It is known to extract metals, especially copper and to a much lesser degree iron, from aqueous solutions containing the metals in the form of, for example, salts, by contacting the aqueous solution with a solution of a solvent extractant in a water immiscible organic solvent and then separating the solvent phase loaded with the metals, i.e. containing at least a part of the metals in the form of a complex. The metals can then be recovered by stripping with a solution of lower pH (the electrolyte) followed for example, by electrowinning. Most commonly, the aqueous metal-containing solutions for extraction are the result of the acid leaching of ores.
Solvent extractants which have found favour in recent years particularly for the recovery of copper from aqueous solutions include oxime reagents, especially o-hydroxyarylaldoximes and o-hydroxyarylketoximes. The oxime reagents exhibit a high degree of selectivity of copper over iron which is commonly expressed as the transfer ratio. The transfer ratio is the ratio of the loaded organic copper concentration minus the stripped organic copper concentration divided by the loaded organic iron concentration minus the stripped organic iron concentration. Although a high transfer ratio is usually desired, the presence of some iron in the electrolyte can also have benefits as described in for example US patent application 2005/0023151. In some cases iron is desired as a counter ion to maintain a certain EMF value in the electrolyte. The selectivity of copper over iron is a function of the metal extractant, the metal and acid concentrations in the leach solution and electrolyte, and the operating conditions in the solvent extraction plant. In many instances using the present copper solvent extractants the selectivity of copper over iron is such that insufficient iron is transferred to the electrolyte via the organic phase to maintain the concentration range required. In such cases iron sulphate is added to the electrolyte to achieve the desired concentration.
Using the solvent extraction process it is common for other impurities to be transferred to the electrolyte by a physical means. Impurities transferred to the strip solution will eventually build up in the circuit and have a negative impact on the electrowinning step. For that reason, operations often bleed a portion of the electrolyte to control the build up of impurities. In those cases, the electrolyte must be replaced with fresh water, acid, and iron (usually as ferrous sulphate). In some cases the amount of iron which must be added to make up for that which is lost in the bleed can be excessive. The addition of iron can negatively effect the economics of an operation. For these reasons it would be highly desirable to have a solvent extractant formulation which would allow an operation to achieve a desired transfer ratio—without losing the well known benefits of the hydroxyl oxime formulations commonly used today.
Although there are many chelating reagents which have a higher affinity for iron than the hydroxy oximes, It has surprisingly been found that the addition of small quantities of a select few iron chelating reagents (hereafter referred to as selectivity modifiers) to oxime reagents has a profound effect on the copper over iron selectivity characteristics of the resulting extractant composition. This effect on the resulting copper:iron transfer ratio is significantly greater than the effect of the sum of the two products when used independently.